We propose to systematically describe the quantitative relations between nitrogen balance and energy balance and the level of nitrogen and calorie intake in surgical patients with nutritional depletion who are undergoing nutritional therapy. Two levels of nitrogen intake, 6.2 and 14 mg N per kcal of measured resting metabolic expenditure, will be studied. In each patient, three levels of calorie intake (0.6, 1.3 and 2.0 times resting metabolic expenditure) will be compared at a constant N intake. Comparisons will be made of the effects of providing glucose or triglycerides as the predominant source of calories. We will also describe changes in hormone concentrations, and concentration and peripheral utilization of amino acids and other key substrates as they relate to the course of nutritional therapy. Methods for estimating total energy expenditure, rather than resting energy expenditure will be developed in order to be able to relate nitrogen balance to energy balance rather than energy intake. The upper limits of nitrogen intake suitable for depleted patients will be investigated. We aim to provide better understanding of metabolic and hormonal changes in nutritional depletion and more quantitative guidelines for nutritional therapy of depleted patients.